Texturization consists in creating roughness at the surface of the material in order to enable multiple reflection of light incident on its surface, thereby leading to greater absorption of the light inside the material. The roughness obtained in this way has two complementary effects: the first effect is to reduce the reflecting power or optical reflectivity of the surface; and the second effect is to increase the length of the optical path travelled by the incident light inside the material. In a photo-cell, the increase of light absorption in the silicon gives rise to an increase in the effectiveness with which light is transformed into electricity.
Several methods make it possible to achieve this result and lead to uniform texturization of the surface revealing well-defined crystallographic planes thereat. Mention may be made of methods such as mechanical engraving, laser etching, photo-lithography, masking, etc., however they are complex and expensive to implement. The method of surface texturization that is presently known and used on an industrial scale consists in using at high temperature an alkaline aqueous solution based on sodium hydroxide (NaOH) or on potassium hydroxide (KOH). Those solutions have the property of etching silicon aniosotropically depending on the crystallographic orientation of the grains situated at the surface, thereby modifying the surface morphology of the silicon. Etching speed is about 100 times greater on planes having crystallographic orientation 100! than on 111! planes. This causes the surface to be texturized in the form of regular pyramids situated on 100! planes that trap incident light and this is referred to by the term "macro-texturization". For multi-crystalline silicon, it is estimated that only 20% of the surface is constituted by grains having this crystallographic orientation, which means that the treatment is less effective with respect to optical reflectivity. The method is therefore fully effective only in the special case of monocrystalline silicon having crystallographic orientation 100! at its surface to be treated.
To reduce the optical reflectivity of an n-type monocrystalline silicon surface to a greater extent, work relating to photo-electrochemical etching in an acid medium has been performed by A. Lagoubi et al. (11th Photo-voltaic Solar Energy Conference, Montreux, 1992). The superposition of a nanoporous outer layer (a layer having pores of diameter less than 50 nm) and of a macro-porous inner layer (pores of diameter greater than 50 nm) is revealed by dissolving the nanoporous layer. The surface coated in the macro-porous layer has lower reflectivity than does the untreated surface.
That work suffers from the drawback of relating solely to n-type monocrystalline silicon. Also, it is not easy to perform a photo-electrochemical method in an industrial medium.
Users therefore need to have available a method that is industrially applicable for achieving uniform texturization of the surface of multi-crystalline silicon regardless of the crystallographic orientation of the grains at its surface.